In a HCCI engine, the fuel and oxidizer are mixed together similarly as they would be in a spark ignition engine (gasoline engine). In contrast to the homogeneous charge spark ignition engine, which uses an electric discharge to ignite a portion of the fuel/oxidizer mixture, a HCCI engine depends upon spontaneous reaction when the density and temperature of the mixture are raised by compression. until the entire mixture reacts spontaneously. This is similar to a stratified charge compression ignition engine (diesel engine) which also relies on temperature and density increase resulting from compression. However, rather than being spontaneous as in the HCCI engine, combustion occurs in a diesel engine at the boundary of fuel-air mixing, caused by an injection event; introduction of fuel into the already compressed oxidizer is what initiates combustion.
In both the homogeneous charge spark ignition and the stratified charge compression ignition (HCSI) engines, the burn starts at one (or possibly a few) place and propagates through the fuel/air mixture. In the gasoline (an engine, the flame initiates at an electrical discharge point and propagates through a premixed homogeneous charge of air and fuel. In the diesel (SCCI) engine the flame starts near the one or more injection points via auto-ignition and propagates through a heterogeneous mixture at the moving boundary of fuel air mixing. Under HCCI conditions, a homogeneous mixture of fuel, air, and residual gasses from previous cycles are compressed until auto-ignition occurs. Combustion initiates substantially simultaneously at multiple sites throughout the combustion chamber and there is no discernable flame propagation.
HCCI engines have a number of advantages: hydrocarbon and CO emissions on par with gasoline engines, efficiency on par with diesel engines, and nitrogen oxide (NOx) emissions that are substantially better than either gasoline or diesel engines. HCCI engines produce no soot and can operate using gasoline, diesel fuel, and many alternative fuels.
A salient aspect of HCCI engines is that the fuel/air mixture burn virtually simultaneously because ignition starts at several places across the cylinder at once. With no direct initiator of combustion, the HCCI process is inherently challenging to control. To enable dynamic operation in an HCCI engine, the control system changes the conditions that induce combustion. Thus, relevant parameters for the engine to control include: the compression ratio, inducted gas temperature, inducted gas pressure, fuel-air ratio, quantity of retained or reinducted exhaust, and blend of fuel types.
Another salient aspect of HCCI engines is that they have a narrow power range because spontaneous ignition occurs around a single designed operating point. An engine having a single operating point is certainly useful in a hybrid vehicle. On the other hand, most applications require an engine to be able to modulate its output to meet fluctuations of demand by an operator. For high load operation, the engine may switched over to operate in a spark ignition (SI) mode, leaving HCCI operation for more moderate load operation.
Due to different characteristics of the HCCI and SI combustions, the in-cylinder ionization signals are quite different, both in magnitude and shape. The ionization signal magnitudes during HCCI combustion is typically more than a factor of ten lower than during SI combustion due to different combustion characteristics (summarized above). As a result, it is very difficult (nearing impossible) to detect ionization current during HCCI combustion mode using an ionization detection circuit that was originally designed for an SI combustion only context.
What is needed is an apparatus for effective detection of ionization signals in an engine that operates in a HCCI mode as well as a SI mode.